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10-09-2015, 18:31   #1
M.T. Cranium
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Heavy rainfall alert: Fri 11th - Mon 14th Sept

I was going to post a LEVEL 1 alert but numerical guidance from several global models are indicating potential for 50 or even 60 mm rainfalls between Friday early morning and Saturday mid-day. Despite the recent run of dry weather, that could be enough rain to produce considerable flooding in low-lying and poorly drained areas and especially on roadways.

The cause of this heavy rainfall is a very slow moving low pressure trough oriented NNW-SSE already making landfall in west Munster then coastal Mayo this evening. Numerical weather prediction is of course not a perfect science but be forewarned, the GFS model indicates 40-60 mm rainfalls possible in parts of central and southeast Connacht and western to central Munster, and amounts of 30-50 mm in most other counties. The GEM model has output that I recognize as top end for their model, whatever their objective number crunch shows, the intensity alone has me concerned given the very slow pace of this system.

It will rain steadily and sometimes very heavily (thundery bursts are possible) for at least 12 to 18 hours once the rain begins in any one location and possibly for 24 hours straight. So the flood potential comes more from that duration than any one hour or two within the system. I am expecting to see reports of 7-12 mm an hour rainfalls and perhaps several consecutive ones.

While almost anywhere in the road network and in towns and cities (in favoured locations) could see flooding at some point, I think that Cork and possibly some towns in Roscommon, east Galway and southeast Mayo would be somewhat more likely to see serious urban flooding problems (underpasses filling up with 3-5 feet of water not out of the question, for example). There won't be much wind as this low is rather weak in terms of pressure gradient, but with new moon approaching on the 13th, tides will be increasing and this could interact with the runoff near the south coast to exacerbate flooding problems.

Will end this thread after a discussion with a map showing actual total rainfalls including those of Boards members, be sure to post some totals (Friday-Saturday) if you can in either this thread or the observations thread.

As this is likely to come up, expect the rain to end Friday night in many places but mid-day Saturday in Dublin-Kildare and Saturday afternoon north of there. The system will tend to "pivot" during Friday night which is why southeast Connacht and far west Leinster would be perhaps subject to higher totals. The smallest totals might be expected in northwest Donegal and southeast Wexford perhaps (25 mm might be the minimum).

Please note that the thread title was upgraded at 1430h Friday to reflect two concerns -- the rainfall event Friday-Saturday will be followed in parts of the west by further heavy rain Sunday-Monday, and total rainfalls over that period (counting what has already fallen to 1430h Friday) may exceed 100 mm over large portions of Kerry, Cork, west Limerick, Clare, Galway and Mayo. This introduces a LEVEL 3 "take action" alert status for a few, still considering that these totals are over 96 hours the runoff into other more populated areas such as Cork city may stay inside the LEVEL 2 criterion but stay in close touch with your local flood authorities for updates through Monday in parts of west Munster and Connacht.

Note: As of 0300h Saturday, most of the rainfall from "round one" of this two-part rainfall extravaganza has moved to the east coast and parts of Ulster where it should gradually wind down later on this morning. A second burst of moderate or possibly heavy rainfall will spread in from the south on Sunday and may peak on Monday in western Connacht (again). This second round is not expected to be as heavy although 30-50 mm could fall in some of the same places that saw 75-125 mm on Friday. Some parts of the east and north are not expected to see much more than 10 mm from round two. The southeast may see 20 mm.

Last edited by M.T. Cranium; 12-09-2015 at 03:17.
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10-09-2015, 18:42   #2
Harry Palmr
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Well the place is starting in get a bit dusty.
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10-09-2015, 19:04   #3
maquiladora
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This may be overdoing it a tad, but the 12Z WRF NMM is showing 150mm over high ground. The Met Eireann criteria for a red warning is +70mm.

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10-09-2015, 19:42   #4
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Euro4 is up into the hundred plus mark too
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10-09-2015, 19:48   #5
M.T. Cranium
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Readers should note that the map is for mid-event in central regions and somewhat before peak potential in Leinster.

This would all suggest extreme caution in driving in hilly areas Friday night in particular when you can't see as well ahead of you, if those rainfall totals were to verify on higher ground, some culverts could be washed out and minor landslides or certainly mudslides would be possible, bad enough if you can see them ahead of you, worse if you're rattling along at 70 km/hr and run into them.

Same goes for flooding of darkened roadways which you could expect to encounter near small stream bridges and lower portions in areas with variable elevation.

It's a big change in the weather too, that often catches people unawares, not like it has been raining off and on for weeks.
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10-09-2015, 20:22   #6
Oneiric 3
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Where the ECM 12z has the heaviest rain tomorrow at 18z. Transfers east during the evening.



GFS 12z has over 70 mm falling in my area over a 24 hour period, ECM has less than half of that. My rain gauge dusted off to test which model will be closer the mark.
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10-09-2015, 20:22   #7
maquiladora
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12Z ICON also showing 100mm in places by Saturday morning.

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10-09-2015, 20:26   #8
Meteorite58
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Rain started her near Tralee at 18.00, 7mm so far with some heavy bursts. Raining lighter atm

Must say didn't expect anything like what is being forecast.

Feeling mild at 15.3C
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10-09-2015, 20:36   #9
Oneiric 3
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Quote:
Originally Posted by maquiladora View Post
12Z ICON also showing 100mm in places by Saturday morning.
YR 12z showing about half that but within the same overall area.



Would not be surprised if these highest totals transferred a little more east in later runs.
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10-09-2015, 20:38   #10
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I am flying into Shannon tomorrow. Will this be problematic ?
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10-09-2015, 20:57   #11
Oscar Bravo
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Tom, there shouldnt be any problems, CATII ILS approaches tomorrow at times, just a horrible wet day
TAF EINN 101700Z 1018/1118 14018KT 9999 SCT015 BKN030
BECMG 1021/1023 -RA BKN013
TEMPO 1101/1118 4000 RA SCT005 BKN008 PROB40
TEMPO 1103/1117 2000 +RA BKN005
BECMG 1108/1110 17008KT
BECMG 1115/1117 31014KT= A mucky day
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10-09-2015, 21:21   #12
Meteorite58
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Tralee: Heavy rain again, 10mm ( checked against the manual gauge )
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10-09-2015, 21:27   #13
Oscar Bravo
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Getting it's act together up along the coast
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10-09-2015, 21:27   #14
M.T. Cranium
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Here's my attempt at a numerical prediction:

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10-09-2015, 21:53   #15
Meteorite58
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Very heavy rain here near Tralee
Rain Rate 25mm per Hour

15mm and counting
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